Low molecular weight phosphonitrilic chloride ("PNC") oligomers can be made by the reaction of ammonium chloride and phosphorus pentachloride in a solvent such as monochlorobenzene at reflux. The reaction can form both cyclic oligomers and linear oligomers. Cyclics are favored by the use of a stoichiometric excess of ammonium chloride and linears are favored by the use of a stoichiometric excess of phosphorus pentachloride. Cyclics contain in the range of 3-7 (PNCl.sub.2) units. The major cyclic is trimer (up to 90% of the reaction product) followed by tetramer (up to 15% of the reaction product).
Linear PNC oligomers are oily products containing about 2-20 (PNCl.sub.2) units. The most abundant linear oligomer contains about 3 to 6 (PNCl.sub.2) units but can contain up to about 50 units and still be an oil.
Both cyclic and linear PNC oligomers can be converted to high molecular weight linear phosphonitrilic chloride by thermal polymerization under controlled reaction conditions. The high molecular weight PNC linears can be substituted in a reaction in which the chlorine atoms are replaced by other substituent groups such as fluoroalkoxy, phenoxy and alkylphenoxy to form polyorganophosphazenes. These organopolyphosphazenes can be used to make articles having fire resistance and low smoke emission. They can be foamed to make thermal insulation or extruded onto wire and cable as electrical insulation.
The low molecular weight PNCs are somewhat unstable and of little use in this form. They can also be substituted with groups such as phenoxy or propoxy to form organophosphazene oligomers which are useful as fire retardants. However, these substituted oligomers are fairly low molecular weight materials and cannot be used in the conventional polymer applications. Therefore a need exists for a method to cross-link the low molecular weight organophosphazene oligomers to convert them to high molecular weight polymers.